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The Great Barrier Reef


Great Barrier Reef a semi-unique mix of platform reefs, fringing reefs and ' ... The large extent of reefs creates 'great barrier' Reef Building Processes ... – PowerPoint PPT presentation

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Title: The Great Barrier Reef

The Great Barrier Reef
  • Reef Formation Age, Reef Building Processes and
    Current Reef Situation

Great Barrier Reef Overview
  • Located in the Coral Sea on the coast of
    Queensland (NE Australia)
  • Largest Reef System in the world consisting of
    3,000 individual reefs surrounding 900 islands
  • 2,600 km long with an area of 344,400 km2
  • Consists of 400 species of hard and soft corals
    and supports 10 of thousands of other species

  • Formation Age
  • Deep core estimate
  • Cause of formation
  • Building Processes
  • Formation of Fringe Reefs
  • Morphologies and Growth Rates of the Great
    Barrier Reef
  • Current Reef
  • Study to determine relationships between coral
    communities and reef building
  • Causes of deterioration

Reef Formation Age
  • Previous age of formation unknown, estimated from
    1 Ma to less than 500 ka.
  • Modern Reef system likely formed on a succession
    of reefs from throughout the Pleistocene
  • New core holes drilled to find more exact dates

Reef Formation Age
  • Core Taken in Two Locations- younger inner reef,
    older outer reef (inner reef is largely ignored)
  • Dates found using a combination of Sr isotope
    stratigraphy, magnetostratigraphy and
    spin-resonance dating.
  • Base of Section 1 dated at 770 /- 280 ka
  • Two samples in Section 2 dated at 600 /- 280 ka
  • One sample in Section 3 dated at 600 /- 280 ka
    as well
  • Reef Formation age is 600 ka

Reef Formation Age
  • Entirety of reef formation occurs after the
    Matuyama-Brünhes magnetostratigraphic change
  • Reef formation begins at approximately the same
    time as the onset of large amplitude saw-tooth
    100 k.y. d18O cycles
  • The Authors believe this means the Reef
    initiation is related to the onset of full
    eccentricity-dominated glacio-eustatic seal level

Reef formation Age Recap
  • Isolated Reef formation begins after 790 ka
  • Full Reef Conditions formed at 600 ka
  • Reef formation possibly related to 100 k.y.
    oxygen isotope variations

Reef Building Processes
  • Reef formation begins with Fringing Reefs
  • Fringing Reefs generally form on sediment or
    bedrock near an island or coastline, but are
    inhibited by an influx of freshwater and river
  • Eventually enough reef is formed to become a
    barrier reef (Evolution of Reef forms
    hypothesized by Darwin)
  • Great Barrier Reef a semi-unique mix of platform
    reefs, fringing reefs and almost barrier reefs
    that are a hybrid between fringing and barrier
    types. The large extent of reefs creates great

Reef Building Processes
  • Fringing Reefs form to fill accommodation space,
    growing up to sea level then outward
  • Where the reef intersects the sea surface a reef
    flat is formed

Reef Building Processes
  • There are two general reef building morphologies
    in the Great Barrier Reef, one in which the
    reef hit sea level at an early age and grew
    mostly laterally (A), and another in which the
    reef grew below sea level for a time and formed
    small islands with debris filled troughs between
    them upon reaching sea level (B)
  • In both cases reef building occurs at a rate of
    1.7-11.8 mm/year vertically to sea level and

Current Reef
  • Study of relationship between coral communities
    and reef development
  • Why is there less development at Broad Sound
    (near 22 South)
  • Natural vs. Human-induced degradation

  • Developed reefs formed of both framework and
    detrital elements into distinctive flats and
  • Least developed reefs are incipient reefs with
    lack reef flats
  • In addition there are coral communities lacking
    framework or detrital accumulations

Area of Study
  • Reef development is stunted near Broad Sound (22
    S), a large shallow, silty embayment with no
    major river running into it
  • Broad Sound is the location of Eastern
    Australias largest tidal range of 10 m
  • Tidal currents suspend fine bottom sediments,
    preventing reef development
  • Reefs to the north and south of the area are
    beyond the influence of these tidal forces

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Location of Study Sites
  • Sites located in four regions
  • Islands classified as inshore or offshore based
    on distance to land, surrounding shelf depth and
    distance from major channels

  • Fringe reefs visited at a number of islands and
    both exposed and sheltered sides were surveyed
  • 10X20 m area within 5 m of reef flat surveyed in
    each case
  • Size and identity of each coral was recorded.
  • Size was divided into 1-10, 11-50, 51-100,
    101-300 and greater than 300 cm classes
  • Species was recorded in most cases, although
    genera was recorded in some highly speciose cases
  • Depth, mean annual tidal range, distance to
    mainland, distance to nearest river, depth of
    surrounding shelf, sheltered or unsheltered and
    island location were recorded for each site

Analytical Methods
  • Correlations between site descriptions and coral
    distribution and abundance were found using
    canonical correspondence analyses
  • A variety of other sortings were used to find
    similarities and differences between dissimilar

  • Regions 1, 2 and 4 had corals to depths of 12 m,
    and region 3 had corals only to 4 m, which
    corresponds to the increased turbidity and
    shallowness of region 3
  • Offshore reefs tended to exhibit more species
    richness, higher colony density, higher soft
    coral cover and lower macrophyte cover

  • Region 1 featured diverse, dense and large corals
  • Region 2 featured diverse but below average
    density corals
  • Region 3 featured small corals with low densities
    except for encrusting corals
  • Region 4 featured generally low density corals
    and was dominated by Acropora

Comparison between Regions 1 and 3 (22 S)
  • Despite different environments, both region 1 and
    region 3 had similar densities of scleractinian
    and alcyonarian corals and soft corals
  • The clear difference that existed was that region
    1 had more slow growing corals and arborescent
    corals and small corals
  • Light Color- Region 1
  • Dark Color- Region 3

Ordination of sites
  • Locations of sites in ordination and outlines of
    each region
  • Penrith Island is in Northern corner of Region 3
    and is mostly reef flat resulting in faunistic
    differences with other regions
  • Region 3 has an abundance of Montipora and a lack
    of Acropora, differentiating it from the other

Ordination of Sites
Coral Communities
  • Coral Communities can be contoured to show their
    nested faunistic relationships
  • There is increasing diversity away from the reef
    in some cases

Environmental Correlations
  • Depth, distance from mainland and esposure were
    significantly correlated with specific species
    and genera
  • Island location and mean annual tidal range
    correlated significantly when size descriptors
    were added
  • Distance from river and shelf depth never had
    better than poor correlation

  • There is clear evidence of a link between coral
    community structure and degree of reef
  • Broad Sound area is sparsely developed and
    consists of encrusting and foliaceous growth
    forms of coral
  • More developed areas tends to have large massive
    and branching coral colonys.

  • Regions 1 and 2 are experiencing reef growth due
    to high settlement densities and large colony
    sizes, especially of massive and branching corals
  • Region 4 is also experiencing growth by
    accumulation of rubble from staghorn Acropora
  • Region 3 is experiencing little reef development
    due to fast growing small corals which die before
    growing large and do not grow densely. In
    addition they are poorly cemented and more easily

  • Net growth is the sum of frame accretion,
    sediment accretion and destruction
  • Not only do large corals create larger frames,
    they are likely to stay in site when broken,
    resulting in more sedimentation
  • Additionally, large corals have a smaller surface
    area to volume ratio, making them less prone to

  • It is likely there has been poor reef development
    in the Broad Sound area due to a high sediment
  • The tidal currents associated with a large tidal
    range are able to keep 89 mg/l of fine sediment
    in suspension
  • Sediment load can coat corals and prevent growth
    and also limits sunlight available for energy

  • Comparing past coral fauna in an area to those
    currently present can be used to help determine
    if current degradation is a change from past reef
    building capacity

Human vs. Natural Reef Degradation
  • A mismatch if reef building capacity could be due
    to anthropogenic causes or due to natural cycles
  • In cases where human activity favors non-reef
    building coral communities it is likely that
    human intervention is a cause for reef
  • In the case of change that seems related to past
    cycles, natural causes are a likely cause
  • Neither of these cases can provide a sure
    distinction between causes, and in some cases
    both causes may be present

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  • Van Woesik, R., Done, T.J., 1997. Coral
    communities and reef growth in the southern Great
    Barrier Reef. Coral Reefs 16,103115.
  • New constraints on the origin of the Australian
    Great Barrier Reef Results from an international
    project of deep coring. Geology, Jun 2001 29
    483 - 486.
  • D.M. Kennedy, C.D. Woodroffe, 2001. Fringing reef
    growth and morphology a review. Earth-Science
    Reviews 57 (2002) 255277

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